Metal powders find extensive use in the production of electrically conductive articles. Whereas a noble metal such as gold is sufficiently noble to resist oxidation, and silver forms oxides retaining some conductivity, less expensive metals such as copper and nickel will be readily oxidized upon exposure to air. In copper or nickel powder of the desired particle size, non-conductive copper or nickel oxide surface layers are quickly formed to the extent that the powders, as commercially available, do not show any significant bulk conductivity.
This problem has been recognized in the art, and it has been proposed to reduce the oxide surface layers just prior to or during the deposition of the metal powder onto a dielectric substrate (U.S. Pat. No. 4,614,837 and EP-A-297,677). These known methods suffer from the inconvenience that immediate further processing is required after the reduction, because the conductive surfaces formed lack a significant resistance to rapid re-oxidation.
Other known methods being more particularly directed to the incorporation of copper or nickel powder into resinous compositions propose the protection of conductive powders by the use of certain antioxidants and/or coupling agents, deposited onto the metal powder or included in the resin composition.
GB-A-2,171,410 discloses polymer compositions containing a metal powder having deposited thereon a combination of an amino compound, such as N-stearyl propylene diamine, and a silane coupling agent, such as 3-aminopropyl trimethoxy silane.
As shown by the comparative experiments hereinbelow, these coated powders as such show a limited storability.
According to U.S. Pat. No. 4,387,115, oxidation stabilized compositions can be obtained by curing a mixture of a metallic copper powder having a copper oxide surface film, a reducing agent containing a substituted or unsubstituted ortho or para dihydroxy benzene ring and a curable resin. The mixture may optionally comprise a chelate forming compound, inter alia aliphatic diamines. It is emphasized in this publication that the reduction of the copper oxide should occur during curing of the resin in order to ensure the ability of the resulting cured product to retain conductivity. Therefore, this publication does not provide any suggestion that the chemical compounds mentioned therein in general terms would show any merits in rendering free copper or nickel powder conductive, let alone resistant to re-oxidation. A comparative experiment hereinbelow shows that the combination of reducing agent and chelating agent effective in Run No. 5 of U.S. Pat. No. 4,387,115 for rendering copper powder conductive when enclosed in a cured resin matrix failed to produce an electrically conductive copper powder in the present process.
U.S. Pat. No. 4,382,981 discloses conductive coating compositions containing copper powder and an organic titanate coupling agent. Whereas the use of such organic titanate coupling agents is an option of a preferred embodiment of the present process, it is shown by the comparative experiments below that the sole use of such coupling agents is not effective in the present process.
U.S. Pat. No. 4,539,041 discloses a method for forming metal powders by reducing the hydroxide or salt of, inter alia, copper or nickel by heating said compound suspended in a polyol. The sizes and shapes of particles, as well their homogeneity, can be controlled in some cases. However, the present process offers a much wider choice of morphology for the conductive powder, since no restrictions on the morphology of the starting metal powder need apply. Using different starting materials, the present processes are clearly distinct from this known method. Whereas it was found that conductive copper powder can be produced from commercial non-conductive powder by heating in ethylene glycol, the powder thus obtained showed unsatisfactory resistance to oxidation. U.S. Pat. No. 4,539,041 provides no hint to the use of the present reagents and processes, which surprisingly produce surfaces of enhanced oxidation stability.
Some commercial copper and nickel powders are available for electronic applications, which are believed to comprise a proprietary protective coating. However, these powders, when tested, showed either no conductivity at the time supplied or rapidly lost conductivity under more severe storage conditions.